Metal films are used to form electrically conductive traces, electrodes, or connections on semiconductor devices, e.g., electronic or optoelectronic devices. Enlarged areas of such films can be employed as bond pads for enabling electrical connection of the semiconductor device to other electrical circuits or devices, e.g., via an electrically conductive wire attached to the bond pad or a component soldered to the bond pad. A common method for forming a metal film on a semiconductor device substrate includes: (i) spatially selective processing of a mask that leaves exposed certain areas of a top surface of the device substrate where the metal film is desired, (ii) deposition of the metal film over the substrate (both masked and unmasked areas), and (iii) removal of the mask from the substrate, along with any metal deposited on the masked areas, leaving behind the metal film deposited on the unmasked areas of the device substrate top surface. Such a process is referred to as a lift-off process. The metal typically is deposited from a vapor (e.g., by evaporation, sputtering, other physical vapor deposition, or chemical vapor deposition), and the deposition process typically exhibits at least partial directionality, in some cases nearly complete directionality. In a directional deposition process, a surface perpendicular to the deposition direction receives the thickest coating, and coating thickness decreases as the surface is tilted farther from that perpendicular orientation. With a highly directional deposition process and a surface parallel to the deposition direction, little or no metal is deposited. An undercut surface (i.e., a surface tilted beyond parallel to the deposition direction and therefore facing at least partly away from the deposition direction) typically would receive little or no metal in a directional process.
Many semiconductor devices include distinct areal regions of the top surface of the device substrate that are at differing vertical positions (i.e., at differing heights). For purposes of the present disclosure and appended claims: (i) horizontal directions are defined as being parallel the device substrate, (ii) the vertical direction is defined as being perpendicular to the device substrate, and (iii) the substrate top surface is defined as the generally horizontal substrate surface on which the electronic or optoelectronic device is formed. For purposes of the present disclosure and appended claims, the terms “device substrate,” “semiconductor substrate,” or “substrate” shall designate all of the materials that make up the device except for the metal film, including, e.g., one or more semiconductors or alloys thereof, or one or more oxides, nitrides, or other dielectric materials. The “substrate top surface,” “device substrate top surface” or “device top surface” is the top surface of the uppermost of those materials, on selected portions of which the metal film is deposited to form the electrically conductive traces, electrodes, or connections; in some examples an additional electrical contact or electrode can also be formed on the substrate bottom surface. Recessed regions of the device substrate top surface can be formed for any suitable reason. In some examples of an electronic or optoelectronic device, a recessed trench is formed between two non-recessed regions, e.g., so as to electrically separate distinct portions of an active semiconductor layer in the device substrate. In some examples of an optoelectronic device, recessed regions are formed on either side of a non-recessed ridge, e.g., so as to provide lateral optical confinement of one or more optical modes supported by a waveguide on the device substrate. For purposes of the present disclosure and appended claims, the terms “recessed” and “non-recessed” are relative terms indicating only that one areal region (the non-recessed region) is at a height greater than the height of another, adjacent areal region (the recessed region). A recessed region is not necessarily the lowest region on the device substrate; distinct recessed regions are not necessarily at the same height; a non-recessed region is not necessarily the highest region on the device substrate; distinct non-recessed regions are not necessarily at the same height. In certain instances, a given areal region of the device substrate can be recessed relative to one adjacent areal region that is at a greater height), and non-recessed relative to a different adjacent areal region that is at a lesser height).
It is common for an electrically conductive metal film to traverse boundaries between adjacent recessed and non-recessed areal regions. To convey electrical signals or currents across the boundary, the metal film must be formed contiguously on the recessed areal region, the non-recessed areal region, and a transition surface joining them, e.g., a substantially vertical surface spanning the height difference. As noted above, common metal deposition processes are at least partially directional, so that simultaneously depositing the metal film on the adjacent, substantially horizontal areal regions and also on the vertical wall joining them can result in differing film thicknesses, and differing electrical properties such as resistance or conductivity, between the horizontal and vertical portions of the metal film. For example, if a highly directional deposition process is employed with a vertical deposition direction 99 (e.g., as in FIG. 1A, in which the vertical transition surface 103 joins the recessed region 101 and the non-recessed region 102 of the top surface of device substrate 100), there might be insufficient metal deposition on the vertical transition surface 103 to enable sufficient electrical conductivity between the metal film portions 91 and 92 on the recessed and non-recessed regions 101 and 102, respectively. One way to mitigate that problem is use of a deposition direction 99 that is not vertical (i.e., θDEP≠0°), and so not parallel to the vertical transition surface 103, thereby enabling metal deposition on the vertical transition surface 103 as well as the horizontal recessed and non-recessed regions 101 and 102. A deposition direction about 45° from vertical (i.e., θDEP≈45°), and with a horizontal projection roughly perpendicular to the areal boundary between regions 101 and 102, would deposit similar metal thickness on both horizontal and vertical portions of the surface (e.g., metal film portions 91/92/93 on respective surfaces 101/102/103, as in FIG. 1B). However, on a typical device there are areal boundaries, and corresponding transition surfaces, at a variety of horizontal orientations, which would receive different thicknesses of deposited metal in given tilted arrangement. A transition surface could even be left completely devoid of metal if it were shadowed by an adjacent non-recessed region.
For example, as shown in FIG. 2A, a non-recessed areal region 102 (e.g., a top surface of a protruding ridge) separates two recessed regions 101 and 104. The top surface of the ridge (i.e., the non-recessed areal region 102), one of its side surfaces that faces a non-vertical deposition direction 99 (i.e., the near-side ridge surface 103), and at least a portion of a first one of the recessed areal regions 101 (the portion adjacent to the near-side ridge surface 103) would all be coated by respective portions 92/93/91 of the metal film, as well as a portion of the second recessed region 104 displaced from the ridge. However, the other side surface of the ridge (i.e., the far-side ridge surface 105) and a portion of the second recessed region 104 (the portion adjacent to the far-side ridge surface 105) would be shadowed by the ridge, and would receive little or no deposited metal. In another example, as shown in FIG. 2B, a recessed areal region 101 (e.g., a trench) separates two non-recessed areal regions 102 and 106. Both non-recessed areal regions 102/106 and at least an upper portion of one side trench surface that faces a non-vertical deposition direction (i.e., the far-side trench surface 103) would be coated by respective portions 92/96/93 or the metal film. If the trench is not too narrow, or if the deposition direction is not too far from vertical, the entire far-side trench wall 103, and at least a portion of the trench bottom surface 101 (i.e., the portion of the recessed areal region 101 adjacent to the far-side trench surface 103) would also be coated by respective portions 93/91 of the metal film. However, the other side surface of the trench (i.e., the near-side trench surface 107) and a portion 101a of the trench bottom surface 101 adjacent to the near-side trench surface 107 would be shadowed, and would receive little or no deposited metal.
In both of those examples, electrical continuity of the metal film across the ridge or trench would be difficult, if not impossible, to achieve. One conventional approach toward mitigating that continuity problem is variation of the deposition direction during the metal deposition process. That can be achieved in a number of ways, typically by rocking the substrate back-and-forth about a horizontal axis, or by so-called planetary rotation of the substrate relative to the deposition source, during the deposition process. In both of those examples, portions of the substrate top surface that are shadowed from metal deposition at certain orientations of the substrate are not shadowed at other orientations. In the ridge or trench examples described above, rocking the substrate back-and-forth about a horizontal axis parallel to the ridge or trench would result in metal deposition on both near-side and far-side surfaces (because those descriptors switch back-and-forth as the substrate rocks), and also on corresponding portions of recessed areal regions adjacent to those side surfaces. However, there are a number of disadvantages associated with rocking or rotating the substrate during the deposition process. One problem is the additional mechanical and process complexity introduced into the deposition process, providing opportunities for misalignment, maladjustment, malfunction, or failure. Another problem is lack of uniformity of the thickness of the metal film (e.g., as in FIGS. 3A and 3B), because some areas are intermittently shadowed during the deposition process while others are not. To achieve sufficiently low resistivity or sufficiently high conductivity of the metal film in intermittently shadowed areas can result in excessive film thickness over areas that are never shadowed. Another problem is reduced quality of the deposited film on those areas that are intermittently shadowed. Voids, fractures, or other defects 913 (e.g., as in FIG. 4A and the electron micrograph of FIG. 4B) occur more frequently when the deposition process is not continuous, and those can increase resistance or decrease conductivity of the resulting film, or even lead to its failure. Perhaps more significantly, the unpredictable nature of the occurrence of such defects can lead to undesirably large variation in resistivity or conductivity of the metal film.